1. Field of the Invention
This invention relates to fabricating a semiconductor wafer and, more particularly, to a retainer used for biasing a wafer clamp against a wafer during processing upon the wafer and for raising the clamp from the wafer after processing.
2. Description of the Related Art
A semiconductor wafer is classified as a monolithic substrate containing both conductive and non-conductive materials. Materials upon the wafer are selectively fashioned across the semiconductor wafer to form an integrated circuit. Each layer of material is patterned according to a technique generally known in the art, and henceforth described as xe2x80x9clithographyxe2x80x9d.
A typical lithography process involves a sequence of steps beginning with a layer of material deposited entirely across the semiconductor wafer. Next, a photoresist (or resist) layer is spin-coated upon the deposited layer. The resist layer may be selectively radiated with, for example, ultraviolet light, electrons, or x-rays. An exposure tool, such as a mask, data tape (in electron beam lithography), etc., is used to selectively expose the resist. Patterns in the resist are formed when the wafer, and specifically the resist, undergoes a subsequent development step. Areas of resist remaining after development protect portions of the deposited layer. The resist which has been removed therefore exposes the underlying material to a variety of additive (e.g., lift-off) or subtractive (e.g., etching) processes that transfer a pattern from the exposure tool onto the conductive or non-conductive material.
There are numerous critical steps involved in accurately and reproducibly placing a pattern upon a semiconductor wafer. Certainly an important step in the expose-develop-etch sequence is the etch step. Accurate etching is of benefit to ensure materials are removed only where they are exposed. While offering high selectivity to only the material being removed, wet etching processing are typically isotropic. That is, wet etching generally attacks the material being removed with the same vigor in all directions. An unfortunate aspect of isotropic etching is undercutting. As film thicknesses decrease, and patterns being transferred become less than, e.g., 2.0 xcexcm, it becomes difficult if not impossible to reliably use isotropic etching. Alternative pattern transfer processes are therefore needed to fabricate integrated circuits with such dimensions.
An alternative pattern transfer mechanism known as dry etching offers the capability of non-isotropic (xe2x80x9canisotropicxe2x80x9d) removal. Dry etching can involve either pure physical removal or a combination of both chemical and physical removal. A popular dry etch process involving both chemical and physical removal is the reactive ion etching (xe2x80x9cRIExe2x80x9d) or reactive ion beam etching (xe2x80x9cRIBExe2x80x9d) technique.
Dry etching utilizes a glow discharge of chemically reactive species (atoms, radicals and ions) from a relatively inert molecular gas. The glow discharge region occurs by biasing a pair of electrodes separated from one another. Typically, one electrode is coupled either to a DC or RF source, while the electrode is typically grounded. The chemically reactive species within the glow discharge react with the material to be etched, leaving a reaction byproduct which is volatile and thereby readily evacuated from between the parallel spaced electrodes. In addition, energetic ions occur within the glow discharge. The ions are directed by the DC or RF field toward a wafer placed upon one of the electrodes arranged between the electrode pair. The energetic ions strike the wafer surface and ablate or sputter remove exposed material from that surface, wherein the exposed material is material residing beneath a space between patterned resist.
As the chemically reactive species adsorb onto the wafer surface, and as ions strike that surface, the surface is somewhat heated. Mechanisms have evolved to allow backside cooling of the wafer as the wafer is being etched on its frontside. Cooling can involve, for example, forwarding a cooling gas, such as helium, across the gap which exists between the wafer backside surface and a wafer pedestal to which the wafer is clamped. Flow of the cooling gas helps maintain a nearly constant temperature gradient across the semiconductor wafer surface during processing. Temperature uniformity is critical to assuring process reproducibility and achieving consistent processing results.
Typical wafer clamps extend across the wafer frontside surface around the edge or periphery of the wafer to seal between the edge to the wafer pedestal. Proper amounts of downward force on the wafer clamp toward the pedestal minimizes or prevents leakage of cooling gas from the wafer backside, through the edge/pedestal gap and into the wafer processing environment. This helps eliminate or significantly mitigate cooling gas interference with critical glow discharge parameters and chemistry.
FIG. 1 illustrates one example of a typical processing tool 10 which can initiate and sustain dry etching and, preferably, ion-assisted etching such as RIE and RIBE. Tool 10 includes a reaction chamber 12 adapted to receive incoming gas 14 through an inlet port. Gas 14 is used to bring about glow discharge during times when parallel-placed electrodes 16a and 16b are powered. Volatile byproducts 18 can be evacuated from the glow discharge area by an outlet port, as shown.
Lower electrode 16b is considered a wafer pedestal in that it accommodates a wafer 20 placed thereon. Various apertures may exist within electrode 16b to allow cooling gas flow across the backside surface of wafer 20. To ensure wafer 20 is sealed against electrode 16b, a wafer clamp, configured similar to a ring, is designed to abut against the upper surface of wafer 20 about the wafer perimeter. Clamp 22 is retained in a movable, biased position relative to upper electrode 16a so that when upper electrode 16a moves, so will clamp 22 to some extent.
FIG. 1 illustrates wafer clamp 22 retained upward away from wafer 20 to allow ingress to wafer 20 during loading and unloading of the wafer into chamber 12. During processing, the gap between electrodes 16a and 16b diminishes, and clamp 22 is forced downward upon the perimeter of wafer 20. Clamp 22 is retained using clamp retainers 24.
FIG. 2 illustrates one example of portions of a dry etch mechanism, a suitable mechanism being that obtainable from LAM Research Corp., model no. 4720, 4600, 9600, etc. The portions shown in FIG. 2 comprise a ring 28 configurable within an upper electrode 16a (shown in FIG. 1). Ring 28 is secured using various attaching devices, or screws 30. Coupled to the underside surface of ring 28 may be a non-reactive plate 32. Plate 32 serves to cover ring 28 and various reactive materials within ring 28 as well as upper electrode 16a. Plate 32 preferably comprises a material which does not react to the glow discharge material between the electrodes, or the gas species 14 introduced into the chamber. According to a metal etch example, plate 32 may comprise a quartz or ceramic material which is substantially inert to metal etchants delivered into the chamber.
Coupled a biased distanced below plate 32 and ring 28 is wafer clamp 22. Wafer clamp 22 appears somewhat like a ring or flat washer having a downward extending flange or lip 34. Lip 34 abuts with the upper surface at the perimeter of a wafer 20. Regions radially outside lip 34 accommodate various apertures 36 circumferentially placed about clamp 22.
Apertures 36 within clamp 22, along with apertures 38 within plate 32, accommodate passage of a retainer 24. A sleeve 40 may be inserted into aperture 36 between the inward facing wall of aperture 36 and retainer 24 to prevent friction between the rigid plate 32 and the rigid material of retainer 24. Preferably, sleeve 40 is made of a Teflon(copyright) substance.
Once sleeve 40 is in place, retainer 24 extends through aperture 38, through washer 42, through aperture 36, and finally through washer 44. Configured on a shaft at one end of retainer 24 is a pair of regularly extending tabs 46. Tabs 46 extend radially outward, yet are smaller than the inside diameter of sleeve 40 and aperture 36. Tabs 46 are designed to fit through the keyhole openings 50 of washers 42 and 44. One-quarter rotation of retainer 24 relative to washer 44 allows retainer 24 to retain or xe2x80x9chold xe2x80x9d clamp 22 a biased distance below upper electrode 16a (shown in FIG. 1). Biasing force upon clamp 22 occurs via a spring 54 which extends between the upper surface of retainer 24 and a cutout within ring 28. When upper electrode 16a is drawn upward, a lip near the upper regions of retainer 24 secures retainer 24 within the recess of ring 28, and washer 44 secures clamp 22 to retainer 28 a spaced distance below plate 32 proportional to the length of retainer 24. However, when the upper electrode moves downward to a processing position, clamp 22 extends about wafer 20, and is biased in the clamping position by spring 54. Further details as to the configuration of retainer 24 is best illustrated in a detailed, cross-sectional view provided in FIG. 3.
FIG. 3 depicts retainer 24 shown biased downward by spring 54. Wafer clamp 22 is retained in position between washers 42 and 44. Tabs 46 are rotated relative to the keyhole openings within the lower washer 44 so that the washer will not fall from its retained position.
Washers 42 and 44 maintain a fixed position between clamp 22 and retainer 24. As the upper electrode moves downward toward the lower electrode, retainer 24 also moves downward along with clamp 22. Downward clamping force can be provided, in part, by spring 54. When the upper electrode moves upward from the lower electrode, retainer 24 moves with the upper electrode, as does wafer clamp 22.
FIG. 4 illustrates in more detail the various aspects of retainer 24 and washer 44 placeable on one end of retainer 24. Formed at one end of retainer is a shaft terminated into a pair of radially extending tabs 46. The tabs are dimensioned smaller than a keyhole opening 50 within washer 44. Keyhole opening 50 is shown as substantially circular, except for two radially extending cutouts 58a and 58b. Cutouts 58a and 58b receive tabs 46, and the circular portion of keyhole 50 receives shaft 56. Two small indents or recesses 60a and 60b exit near keyhole 50 to retain tabs 46 after they are rotated one quarter turn from the insertion cutouts 58. Cutouts or recesses 60 are preferably into only the downward-facing surface of washer 44 as shown.
Tabs 46 are relatively small in order to fit through apertures and/or keyholes less than one half inch in diameter, for example. In some instances of, for example, a LAM Research, Corp. dry etcher such as a model Rainbow tungsten etcher, tabs 46 extend less than one eighth inch from the outer surface of the shaft. The relatively small tabs are, unfortunately, highly susceptible to breakage at the point in which they connect to the outer surface. The quartz or ceramic wafer clamp 22 is generally quite heavy and is supported almost entirely by tabs 46 when the upper electrode is moved to xe2x80x9chomexe2x80x9d position to allow wafer loading and unloading. Periodically, the wafer clamp must be cleaned, or removed and replaced with another wafer clamp. To remove wafer clamp 22 from retainers 24, washer 44 is rotated so that protrusions 58a and 58b align with tabs 46. This allows washer 44 to be removed, along with clamp 22 from shaft 56 of retainer 24. Any misalignment of tabs 46 to the apertures 36 within clamp 22 will cause tabs 46 to break when drawn through aperture 36.
Wafer clamp 22 must periodically be removed for cleaning. The clamp is typically made from quartz or ceramic and can be quite heavy as compared to the strength of the portions of retainers 24 used to hold clamp 22. Removal and replacement of clamp 22 can therefore result in breakage of one or more of retainers 24. When used in a metal etcher, retainer 24 is typically made of ceramic or quartz which is known to be somewhat expensive. For example, an average cost of one ceramic retainer 24 is between $150.00 to approximately $350.00. There are generally several retainers which extend through apertures within clamp 22. According to several designs, approximately six retainers 24 are needed to retain the wafer clamp. If occurs on one retainer, typically breakage occurs on all six, leaving a total cost of between $900.00 to over $2,000.00 during each maintenance cycle on the wafer clamp. It is estimated that the wafer clamp must be cleaned or replaced after as few as 20 wafer runs upward to several hundred wafer runs. Thus, the cost of replacing the retainers translated to each wafer being processed can be quite significant.
It is therefore desirable to derive a wafer clamp retainer which is not susceptible to breakage during routine maintenance. It is further desirable to design a retainer which can be easily manufactured at a relatively low cost.
The problems outlined above are in large part solved by an improved wafer clamp retainer hereof The retainer avoids radially extending tabs as may exist in conventional designs, and therefore avoids the problems of breakage associated with the placement of those tabs relative to the retainer shaft. The present retainer includes a shaft at one end of the retainer, and a feature formed about the shaft which is altogether dissimilar from tabs in both structure and function. The feature preferably extends around the entire shaft circumference, and is formed so that it cannot e easily broken or dislodged from the shaft. Thus, instead of having, e.g., two radially extending tabs limited to two localized areas at the end of the shaft, the presently defined feature exists on the outer surface of the shaft between the end of the shaft and the opposing end of the retainer. The feature is therefore spaced from the shaft end where rigidity is suspect. The feature is defined as any indent or protrusion which extends as a ring or spiral about the outer)surface of the shaft.
Designed to be mated with the feature upon the shaft, is an improved washer design. Instead of the washer being conventionally rigid, the present washer is flexible. Specifically, the present washer includes an opening extending entirely through the washer. The opening can be re-shaped when the washer is flexed so that the opening can extend over a shaft outer surface which is larger than the unflexed opening. A flexible washer thereby provides a radially inward biasing force of the opening walls against the shaft outer surface. The amount of bias is dictated by the difference in size between the shaft and an unflexed opening. Preferably, the difference is designed so that, in addition to mating of the features, the opening walls securely affix to the shaft outer surface until time comes when an operator can forcibly remove the washer from the shaft. It is desirous that the combination of improved retainer and washer allows secure coupling and discoupling therebetween but without risk of breaking the retainer and/or washer during routine maintenace.
According to one exemplary embodiment, the feature is an indent or protrusion extending as a ring about the shaft outer surface. The indent or protrusion is spaced from the end of the shaft so that sufficient surface area exists above and below the ring to enhance its strength with respect to the shaft. It is believed that if the feature was placed at the end of the shaft similar to tabs in conventional designs, the feature would be more susceptible to breakage from that end.
According to another exemplary embodiment, the feature is an indent or protrusion extending as a spiral, or xe2x80x9cthreadxe2x80x9d. The thread, or threads, extend a spaced distance from the end of the shaft upward along the shaft outer surface toward the opposing end of the retainer.
Regardless of the feature configuration, a mating washer is provided. The washer contains a counterpart feature which mates with the singular or multiple rings, or threads upon the shaft. The features within the washer are confined to the inward facing wall which surrounds the washer opening. Those features enhance the retention force of the washer upon the shaft so that the retainer or retainers can lift a substantially heavy wafer clamp.